1. Field of the Invention
The present invention relates to contacts material for the make-and-break electrodes in vacuum circuit breakers, etc., that require outstanding current chopping and voltage-withstand characteristics.
2. Description of the Related Art
Existing vacuum valve contacts are constituted from various materials in order to support and improve the current chopping characteristic, anti-arc erosion resistance characteristic, contact resistance characteristic and temperature rise characteristic in addition to the three basic requirements represented by the anti-welding characteristic, voltage withstand characteristic, and current interrupting characteristic. However, it has been considered impossible to satisfy these requirements adequately with one element alone since mutually conflicting materials properties are often required. Contacts materials for specific applications such as high current breaking application, high withstand voltage application or low current-chopping application have therefore been developed by forming material composites or by cladding, etc., and as they stand, such materials exhibit outstanding characteristics.
Copper-bismuth (Cu--Bi) and copper-tellurium (Cu--Te) alloys containing not more than 5 weight % of an anti-welding component of bismuth (Bi) or tellurium (Te) as described in Japan Patent publication No.41-12131(koukoku) and Japan Patent publication No.44-23751(koukoku), for example, are known as contacts materials for high current breaking application that meet the aforesaid three basic requirements.
However, the reason why these have excellent high current interrupting characteristics is that the brittle bismuth precipitated at the particle boundary in copper-bismuth alloy and the brittle Cu.sub.2 Te intergranular and transgranular precipitate in copper-tellurium alloy embrittle the alloy itself, realizing low-weld tripping capability. Likewise, copper-chromium(Cu--Cr) alloy is known as a high withstand voltage and high current breaking contacts material satisfying the three basic requirements. Since there is little vapor pressure difference between its constituents, copper-chromium alloy has the merit that it can be expected to exhibit uniform performance, and depending on how it is used, it is superior to copper-tellurium alloy.
At the same time, silver-tungsten carbide (Ag--WC) alloy (silver 40%) is known as a low chopping current contacts material, as described for example in Japan Patent Application No.42-68447. The alloy is widely used because it displays outstanding low chopping current performance by virtue of the synergistic effect between the thermionic emission of tungsten carbide (WC) and the moderate vapor pressure of silver (Ag).
Even higher performance could be secured from vacuum circuit breakers if further improvements were made in respect of the two problems noted hereunder. One is that, when current is interrupted using an inductive circuit under motor load, etc., without making proper allowance for the vacuum valve, a transient abnormal surge voltage can arise, adversely affecting the integrity of insulation of the load equipment.
The cause of the abnormal surge voltage is the current chopping that occurs at low current when current is interrupted in vacuum (when current interruption is performed forcibly without waiting for the natural zero point in the a.c. voltage waveform). The abnormal surge voltage Vs is proportional to the surge impedance Zo of the circuit and the chopping current Ic. Accordingly, as one means of holding down the abnormal surge voltage Vs, the chopping current Ic must be reduced, and silver-tungsten carbide alloy is utilized as a contacts alloy to secure advantages in this respect.
The other problem is that flashover may occur in the vacuum valve in vacuum circuit breakers after current interruption, giving rise to a phenomenon whereby through-conduction is re-established between the contacts (with non-continuation of discharge thereafter). The phenomenon is called restrike and although the mechanism thereof has not been elucidated, abnormal voltage is apt to develop owing to a sudden reversion to through-conduction once the electrical circuit has reached current interruption status.
According to experiments in which restrike is created by the breaking of a capacitor bank with a circuit breaker using silver-tungsten carbide alloy, the development of an extremely large overvoltage and an excessively large high frequency current is observed. The development of technology for suppressing restrike is therefore being pursued for silver-tungsten carbide alloy.
Although the mechanism responsible for restrike in silver-tungsten carbide alloy is still unknown, experimental observations by the inventors have indicated that restrike occurs with a fairly high frequency between contact and contact, and between contact and arc shield, in the vacuum valve. The inventors have therefore identified highly effective art for suppressing restrike, for example art for inhibiting the abrupt gas released when a contact receives an arc, art for optimizing contact surface form, etc., thereby contributing to the suppression of restrike.
Thus, the inventors made detailed observations on the correlation with restrike of the total amount of gas, the gas species and the form of emission of the gas released in heating silver-tungsten carbide alloy and discovered that the incidence of restrike rises at contacts for which a large amount of gas is released abruptly in pulses, albeit for an extremely short time, near the melting point.
The incidence of restrike was therefore reduced by excluding the factor of abrupt gas release beforehand, e.g. by heating the silver-tungsten carbide alloy above the melting point of silver (Ag), or by improving the sintering technology to suppress pore formation or structural segregation in the silver-tungsten carbide alloy. However, the need for further improvement is recognized in regard to recent requirements for greater suppression of restrike, and it is important to develop other approaches.
Thus, a prominent trend in recent years has been toward increasing severity of consumer operating conditions and diversification of load, with wider adaptation to reactor circuits and capacitor(condenser) circuits. Demand has grown for the provision of even lower chopping current and even lower restrike from low chopping current silver-tungsten carbide alloy, and the associated development and improvement of contacts materials have become a matter of urgency. In particular, because two to three times the normal voltage is applied, the surface of the contacts is greatly damaged by arcing during current breaking and current switching in condenser circuits; as a result, the contacts are vulnerable to surface roughening and ablation, which could contribute to restrike, and for this reason the contacts must be made more resistant to erosion. However, despite the importance of elucidating restrike from the perspective of improving product reliability, neither the technology to prevent restrike nor the direct causes thereof have yet been ascertained.
Although silver-tungsten carbide alloy has been deployed as a low chopping current type contacts material in preference to the aforementioned copper-bismuth alloy, copper-tellurium alloy or copper-chromium alloy, the fact remains that it cannot be considered a satisfactory contacts material given the growing need for lower restrike. Thus, even with the silver-tungsten carbide alloy hitherto preferentially used as low chopping current type contacts material, restrike is still observed in the more demanding high voltage region and in circuits associated with inrush current. It is therefore desirable to develop a contacts material that in particular has outstanding current chopping and anti-restrike characteristics in addition to supporting the aforementioned three basic requirements at an acceptable level.